Transformer drive for low conduction loss rectifier in flyback converter

ABSTRACT

A flyback converter involves a bipolar transistor (BJT) and a parallel-connected diode as the rectifying element in the secondary side of the converter. The transformer of the converter has a primary winding, a first secondary winding, and a second secondary winding. A first end of the first secondary winding is coupled to the BJT base. A first end of the second secondary winding is coupled to the BJT collector and to the anode of the diode. The first and second secondary windings are wound such that when primary winding current stops, pulses of current flow out of the first ends of the first and second secondary windings. These currents are such that the BJT is maintained in saturation throughout at least most of the time current flows through the rectifying element, thereby achieving a low forward voltage across the rectifying element, reducing conduction loss, and increasing converter efficiency.

TECHNICAL FIELD

The described embodiments relate to rectifiers, and more particularly torectifiers for flyback power supplies.

BACKGROUND INFORMATION

FIG. 1 (Prior Art) is a simplified circuit diagram of a flybackconverter power supply 1. Flyback converter 1 generates a 5.0 volt DCvoltage from a 110 volt AC source 2. 110 volts AC supplied from source 2is present between connectors 3 and 4. The 110 volt AC voltage isrectified by a full wave bridge rectifier comprising diodes 5-8.Capacitor 9 is a smoothing capacitor. A rough DC voltage V_(IN) ofapproximately the 150 volt peak voltage of the 110 volt AC RMS inputsignal is present on conductor and node 10. A switch 11 is rapidlyswitched on and off to pull pulses of current through the primarywinding 12 of a transformer 13 from this V_(IN) conductor. When a pulseof current is pulled through the primary winding 12, an amount of energyis stored in the transformer 13. When the switch 11 is then opened, apulse of current is made to flow from the secondary winding 14 so thatenergy stored in the transformer 13 is transferred to the load 15. Thecurrent from the secondary winding 14 flows through a rectifier diode16. Such pulses of current keep charge on a storage capacitor 17 so thatthe desired 5.0 volts DC is maintained across load 15 between V_(0uT)conductor 18 and ground conductor 19. Standard sensing and controlcircuitry that controls the switching of switch 11 is not illustrated inorder to simply the diagram. The flyback topology of FIG. 1, includingits sensing and control circuitry, is well known in the art.

FIG. 2 (Prior Art) is a set of simplified waveform diagrams. Thesediagrams set forth waveforms of voltages and currents present in thecircuit of FIG. 1. The upper waveform labeled V_(S) shows the voltagepresent across switch 11. From time t₀ to time t₁, switch 11 is closed.Current is flowing from node 10, through the primary 12, and throughswitch 11, and to ground node and conductor 20. From time t₀ to time t₁this current increases as illustrated in the waveform labeled I_(S).From time t₀ to time t₁, energy is being stored in the transformer.Switch 11 is closed. Accordingly, the voltage across switch 11 is zero.Magnetic flux is building in the transformer as indicated by thewaveform labeled “MAGNETIC FLUX”. Then at time t₁, switch 11 is opened.The opening of switch 11 causes a current to stop flowing in the primarywinding 12, and to start flowing in the secondary winding 14. Asillustrated in the fourth waveform labeled I_(D), this current flowingin the secondary decreases over time. The magnetic flux in thetransformer decreases as well. At time t₂, there is no more energystored in the transformer and the secondary current stops flowing. Fromtime t₂ to time t₀, there is little or no current flow in either theprimary or the secondary windings of the transformer as indicated by theI_(S) and I_(D) waveforms. The switching cycle repeats at time t₀ whenswitch 11 is closed again to start the next cycle. The switching periodfrom time t₀ of one period to time t₀ of the next period may, forexample, be ten microseconds.

FIG. 3 (Prior Art) illustrates current flow from time t₂ to time t₀.Reference numeral 21 identifies the split core of transformer 13. FIG. 4(Prior Art) illustrates current flow from time t₀ to time t₁. FIG. 5(Prior Art) illustrates current flow from time t₁ to time t₂.

When current is flowing from the secondary winding 14 of the transformer13 and to capacitor 17 and load 15, the current is flowing throughrectifier diode 16. The rectifier diode 16 being in the current pathresults in unwanted power dissipation. At a given time, theinstantaneous power dissipated in rectifier diode 16 is the product ofthe instantaneous current flow through the diode and the instantaneousvoltage being dropped across the diode. Average power dissipation inrectifier diode 16 is the average of such instantaneous powerdissipation taken over the entire switching cycle of the flybackconverter 1. In a common conventional flyback converter that outputs 20amperes at 5.0 volts DC such as the flyback converter illustrated inFIG. 1, the forward voltage drop V_(F) of the rectifying diode at itsrated current flow is approximately 1.0 volts. Average power dissipationin the rectifying diode may be approximately 15 Watts.

SUMMARY

A Low Forward Voltage Rectifier (LFVR) in an easy-to-employ two-terminalTO-247 package comprises a first package terminal, a second packageterminal, a bipolar transistor, a parallel diode, and a base currentinjection circuit. The collector of the bipolar transistor is coupled tothe first package terminal. The emitter of the bipolar transistor iscoupled to the second package terminal. The parallel diode is coupledbetween the collector and the emitter of the bipolar transistor so thatthe anode of the diode is coupled to the collector and the cathode ofthe diode is coupled to the emitter. The base current injection circuitinjects a base current into the bipolar transistor in forward biasconditions (conditions in which the voltage on the first packageterminal is greater than the voltage on the second package terminal)such that the voltage drop from the first package terminal to the secondpackage terminal is substantially less than 0.7 volts when a currentgreater than a predetermined current is flowing from the first packageterminal to the second package terminal. The forward voltage drop fromthe first to second package terminals may be approximately 0.1 volts.

In one example, if current flow from the first package terminal to thesecond package terminal is less than the predetermined current underforward bias conditions, then the voltage drop from the first packageterminal to the second package terminal is limited by the diode to beapproximately 0.8 volts. In reverse bias conditions (conditions in whichthe voltage between the first package terminal and the second packageterminal is negative), the LFVR blocks current flow.

In one example, the base current injection circuit involves a currenttransformer. The current transformer has a first winding and a secondwinding wrapped around a ring-shaped ferrite core. The currenttransformer, the bipolar transistor, and the parallel diode areinterconnected such that the second winding is in the current path ofthe collector current of the bipolar transistor. The first winding iscoupled to supply a base current to the base of the bipolar transistor.In one specific case, the base current I_(B) supplied by the firstwinding of the current transformer to the base is approximately onethird of the collector current I_(C) flowing through the second windingof the transformer. For collector currents greater than thepredetermined critical collector current I_(C-CRIT), the base currentsupplied to the bipolar transistor is adequate to keep the transistor insaturation such that V_(CE) is substantially less than 0.7 volts(approximately 0.1 volts).

In one example, the bipolar transistor is a Reverse Bipolar JunctionTransistor (RBJT) and the parallel diode is a distributed diode. Boththe RBJT and the distributed diode are parts of a novel RBJT/diodeintegrated circuit. The reverse breakdown voltage from the emitter ofthe RBJT to the base of the RBJT is greater than 20 volts. The reversebreakdown voltage from the emitter of the RBJT to the collector of theRBJT is greater than 20 volts. The bipolar transistor involves manysubstantially square-shaped N-type collector regions disposed in aregular array of rows and columns. Each N-type collector region has oneand only one hole. This hole is a central axial hole so that P-typematerial from an underlying P-type region extends up into the hole. Acollector metal electrode covers the central hole thereby forming adiode contact at the top of the hole. When the distributed diodeconducts, current flows from the collector electrode, down through themany central holes in the many collector regions, through correspondingPN junctions beneath the holes, and to an emitter electrode disposed onthe bottom side of the integrated circuit. The PN junctions togethercomprise the distributed diode. The base metal electrode forms atwo-dimensional grid. Metal of the base metal electrode surrounds theperiphery of each N-type collector region (when considered from atop-down perspective). The collector metal electrode is structured tocontact the N-type collector regions. The collector metal electrodecontacts one N-type collector region, bridges up and over interveningportions of the base electrode between N-type collector regions, andextends down to contact the neighboring N-type collector region. Thevarious neighboring N-type collector regions are interconnected in thisway. This bridging collector electrode structure results in a lowcollector-to-emitter forward voltage when the RBJT is on and conductive.

Using the Low Forward Voltage Rectifier for the rectifying component ina flyback converter power supply reduces average power dissipation ascompared to using a conventional silicon diode for the rectifyingcomponent. Reducing average power dissipation increases power supplyefficiency. The easy-to-employ two-terminal TO-247 package of the LFVRallows a conventional diode in the secondary of a flyback converter tobe removed and replaced with the LFVR with a minimal amount of PCBlayout and power supply design changes.

In another novel aspect, a flyback converter involves a RBJT and adiode. An anode of the diode is coupled to the collector of the RBJT. Acathode of the diode is coupled to the emitter of the RBJT. The emitterand the cathode of the diode are coupled to an output node through whicha storage capacitance is charged. The RBJT and the diode serve as therectifying element in the secondary side of the flyback converter.

The flyback converter further comprises a transformer having a primarywinding, a first secondary winding, and a second secondary winding. Afirst end of the first secondary winding is coupled to the base of thebipolar transistor. A first end of the second secondary winding iscoupled to the collector of the bipolar transistor and to the anode ofthe diode. The first and second secondary windings are wound such thatwhen current flow in the primary winding is stopped during a switchingcycle of the flyback converter, pulses of current flow out of the firstends of the first and second secondary windings. These currents are suchthat the BJT is maintained in saturation throughout at least most of thetime current flows through the rectifying element, thereby achieving alow forward voltage across the rectifying element, reducing conductionloss, and increasing converter efficiency. Several different transformerstructures can be used.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (Prior Art) is a diagram of a conventional flyback converterpower supply.

FIG. 2 (Prior Art) is a waveform diagram showing voltages and currentsin the circuit of FIG. 1.

FIG. 3 (Prior Art) illustrates current flow in the conventional flybackconverter of FIG. 1 from time t₂ to time t₀.

FIG. 4 (Prior Art) illustrates current flow in the conventional flybackconverter of FIG. 1 from time t₀ to time t₁.

FIG. 5 (Prior Art) illustrates current flow in the conventional flybackconverter of FIG. 1 from time t₁ to time t₂.

FIG. 6 is a flyback converter that includes a Low Forward VoltageRectifier (LFVR) in accordance with one novel aspect.

FIG. 7 is a diagram of the IV characteristic of a silicon diode.

FIG. 8 is a diagram of the IV characteristic of a Schottky diode.

FIG. 9 is a diagram of the V_(CE) to I_(C) characteristic of a bipolarconventional BJT, provided that an adequately large base current issupplied to the BJT.

FIG. 10 is a diagram of the V_(CE) to I_(C) characteristic of a ReverseBipolar Junction Transistor (RBJT), provided that an adequately largebase current is supplied to the RBJT.

FIG. 11 is a table that sets forth V_(F) and V_(T) for various differenttypes of rectifying components.

FIG. 12 is a top-down schematic diagram of a square portion of anintegrated circuit in accordance with one novel aspect. The integratedcircuit includes both the RBJT and the distributed parallel diode inintegrated form.

FIG. 13 is a cross-sectional side view taken along sectional line A-A ofthe square of FIG. 12.

FIG. 14 is a diagram of the fly-back converter of FIG. 6, but with aspecific implementation of the LFVR.

FIG. 15 is a set of waveform diagrams that shows voltages and currentsin the fly-back converter of FIG. 14.

FIG. 16 illustrates current flow in the circuit of FIG. 14 from time t₂to time t₀.

FIG. 17 illustrates current flow in the circuit of FIG. 14 from time t₀to time t₁.

FIG. 18 illustrates current flow in the circuit of FIG. 14 from time t₁to time t₂.

FIG. 19 is a simplified cross-sectional diagram of a conventionalbipolar junction transistor (BJT) structure.

FIG. 20 is a simplified cross-sectional diagram of the RBJT of FIG. 14.

FIG. 21 is a waveform diagram showing the voltage drop across andcurrent flow through a conventional diode if the conventional diode isused for the rectifying component in a flyback converter.

FIG. 22 is a waveform diagram showing the voltage drop across andcurrent flow through the LFVR of FIG. 14 between time t₁ and time t₂.

FIG. 23 is a table that compares the average power dissipation of theconventional flyback converter circuit of FIG. 1 involving aconventional silicon diode as the rectifying component to the averagepower dissipation of the novel flyback converter circuit of FIG. 6involving an LFVR as the rectifying component.

FIG. 24 is a detailed diagram of a particular implementation of the LFVRof FIG. 14.

FIG. 25 is a simplified perspective conceptual diagram of the currenttransformer of the LFVR of FIG. 14.

FIG. 26 is a more detailed perspective diagram of the currenttransformer of the LFVR of FIG. 14.

FIG. 27 is a simplified perspective view of the packaged LFVR of FIG. 14before encapsulation.

FIG. 28 is a simplified perspective view of the packaged LFVR of FIG. 14after encapsulation.

FIG. 29 is a simplified flowchart of a method 300 in accordance with onenovel aspect.

FIG. 30 is a more detailed top-down diagram of the RBJT/diode integratedcircuit 78 of FIG. 14.

FIG. 31 is a diagram of the rectangle portion 122 of the integratedcircuit 78 of FIG. 30 taken along certain sections.

FIG. 32 is a diagram of the rectangle portion 122 of the integratedcircuit 78 of FIG. 30 taken along certain sections.

FIG. 33 is a diagram of the rectangle portion 122 of the integratedcircuit 78 of FIG. 30 taken along certain sections.

FIG. 34 is a table that sets forth doping concentrations in the variousregions and layers of the structure of the RBJT/diode integrated circuitof FIG. 30.

FIG. 35 is a simplified flowchart of a method 400 in accordance with onenovel aspect.

FIG. 36 is a simplified circuit diagram of a flyback power supplyconverter 500 in accordance with another novel aspect.

FIG. 37 is a diagram of one example of the transformer of the flybackconverter of FIG. 36.

FIG. 38 is a more detailed diagram of the transformer of FIG. 37.

FIG. 39 is a diagram of an example of the flyback power supply of FIG.36 in which the primary winding, the first secondary winding, and thesecond secondary winding are all wrapped around the center branchportion of the transformer core.

FIG. 40 is a diagram of another example of the flyback power supply ofFIG. 36 in which the transformer functionality includes twotransformers.

FIG. 41 is a flowchart of a method 600 in accordance with another novelaspect.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

FIG. 6 is a simplified circuit diagram of a flyback converter 50 inaccordance with one novel aspect. Flyback converter 50 generates a 5.0volt DC (direct current) voltage from a 110 volt AC (alternativecurrent) source (110VAC RMS). 110 volts AC RMS supplied from source 52is present between connectors 53 and 54. The 110 volt AC voltage isrectified by a full wave bridge rectifier comprising diodes 55-58.Capacitor 59 is a smoothing capacitor. A somewhat rough DC voltageV_(IN) is present on conductor and node 60. The magnitude of V_(IN) isapproximately the peak voltage of the 110 VAC RMS signal, which is 150volts. A switch 61 is rapidly switched on and off to pull pulses ofcurrent through the primary winding 62 of a transformer 63. When onesuch pulse of current is pulled from node 60, through the primarywinding 62, through switch 61, and to ground node and conductor 51, anamount of energy is stored in the transformer 63. When switch 61 is thenopened current stops flowing in the primary winding 62, but a pulse ofcurrent is then made to flow from the secondary winding 64 so thatenergy previously stored in the transformer 63 is then transferred tothe load 65. The current from the secondary winding 64 flows through anovel Low Forward Voltage Rectifier 66 (LFVR). Such pulses of secondarycurrent keep a storage capacitor 67 charged so that the desired 5.0volts DC is maintained across load 65 between V_(0UT) conductor 68 andground conductor 69. Sensing and control circuitry that controls theswitching of switch 61 is not illustrated in order to simply thediagram. Many suitable sensing and control circuits for controllingswitch 61 are known in the art.

In the illustrated example, LFVR 66 comprises a first package terminal70, a second package terminal 71, a base current injection circuit 72, aReverse Bipolar Junction Transistor (RBJT) 73, and a parallel diode 74,interconnected as illustrated in FIG. 6. The anode 75 of the diode 74 iscoupled to the collector of RBJT 73. The cathode 76 of the diode 74 iscoupled to the emitter of RBJT 73. The voltage from the first packageterminal 70 to the second package terminal 71 is denoted V_(LFVR). Thecurrent flow from the first package terminal 70 to the second packageterminal 71 is denoted I_(LFVR). The base current injection circuit 72functions to inject an adequate base current I_(B) into RBJT 73 so thatwhen current I_(LFVR) is flowing the voltage drop (from terminal 70 toterminal 71) is substantially lower than 0.7 volts (for example, about0.1 volts) throughout as much of the time from time t₁ to time t₂ aspossible.

FIG. 7 is a diagram showing the IV curve for an ordinary silicon diodewhere the junction is a semiconductor-semiconductor junction. The diodestarts to conduct current when a positive voltage of approximately 0.7volts (denoted V_(T)) is present from its anode to its cathode. When apositive voltage is present across the diode from its anode to itscathode, and the diode is conducting at its rated current, then theforward voltage drop across the diode is about 1.0 volts. This voltagedrop is denoted V_(F). When the diode is reverse biased (a negativevoltage is present from its anode to its cathode), then the diodeeffectively blocks current flow for negative voltages that are not toohigh. This is the type of diode commonly used for the rectifier diode ina conventional flyback converter.

FIG. 8 is a diagram showing the IV curve for another type of diodereferred to as a Schottky diode where the junction is ametal-semiconductor junction. As shown, the Schottky diode begins toconduct current at a lower positive voltage V_(T) of 0.4 volts betweenits anode and its cathode. The Schottky diode has a lower forwardvoltage V_(F) of 0.7 volts at the rated current of the diode.

FIG. 9 is a diagram showing the collector-to-emitter voltage V_(CE) dropacross an ordinary bipolar transistor as a function of collector currentI_(C). The curve of FIG. 9 assumes that the base current I_(B) isadequately large to keep the transistor in saturation. In the example,the base current I_(B) is one tenth of the rated collector current. Notethat the voltage drop V_(CE) us about 1.0 volts at the rated collectorcurrent.

If the silicon diode of FIG. 7 were used as the rectifier component inthe secondary of a flyback converter, then there would be a 1.0 voltagedrop across the diode when a pulse of secondary current is flowingthrough the diode at the diode rated current. This voltage drop wouldcorrespond to a high power dissipation. Similarly, if the Schottky diodeof FIG. 8 were used as the rectifier component in the secondary of aflyback converter, then there would be a 0.7 voltage drop across thediode when a pulse of secondary current is flowing through the diode atthe diode rated current. This voltage drop would correspond to anundesirably high power dissipation as well. Similarly, if the ordinarybipolar transistor of FIG. 9 were used as the rectifier component in thesecondary of a flyback converter, then there would be a 1.0 V_(CE)voltage drop across the transistor when the pulse of secondary currentis flowing through the transistor at the rated current. This 1.0 voltV_(CE) voltage drop would correspond to a high power dissipation.

FIG. 10 is a diagram showing the collector-to-emitter voltage V_(CE)drop across RBJT 73 of the Low Forward Voltage Rectifier (LFVR) 66 ofFIG. 6, where the base current I_(B) is maintained at one third of thecollector current I_(C). RBJT 73 starts conducting collector currentI_(C) at a collector-to-emitter voltage V_(CE) of about 0.7 volts. Asthe collector current increases, the V_(CE) across RBJT 73 increases upto about 0.9 volts for very low collector currents. As the collectorcurrent increases further, however, the V_(CE) across the transistordecreases rapidly. For a collector current equal to one third of therated collector current I_(RATED), the V_(CE) is less than 0.1 volts.Further increases in collector current I_(C) up to the RBJT ratedcollector current I_(RATED) only slightly increases the V_(CE) acrossthe RBJT. For a collector current equal to the rated collector currentI_(RATED), the V_(CE) is approximately 0.1 volts (denotes forwardvoltage V_(F)) as illustrated. In the illustrated example, I_(RATED) is30 amperes.

FIG. 11 is a table that sets forth the forward bias voltages V_(T) wherethe various rectifiers begin to conduct forward current. FIG. 11 alsoshows the voltage drops V_(F) across the various rectifiers when therectifiers are conducting at their rated currents.

FIG. 12 is a simplified top down diagram of a square portion 77 of RBJT73 and parallel diode 74. RBJT 73 and parallel diode 74 are integratedto be parts of the same RBJT/diode integrated circuit 78. The squareportion 77 illustrated in FIG. 12 is replicated many times in rows ofadjacent squares and columns of adjacent squares across the integratedcircuit 78. From a top-down perspective, the base contact 79 has atwo-dimensional grid structure of horizontally extending metal strips ofthe base contact and vertically extending metal strips of the basecontact. Within each of the squares formed by this two-dimensional gridstructure is a square N-type collector region. RBJT/diode integratedcircuit 78 involves approximately one hundred copies of the squareillustrated in FIG. 12.

FIG. 13 is a cross-sectional side view taken along sectional line A-A inFIG. 12. A base metal electrode 80 makes electrical contact with P-typelayer 81 at base contact 79. A part 82 of the P-type layer 81 serves asthe P-type base region of RBJT 73, and another part 83 of the P-typelayer serves as the P-type anode of a PN junction 84. In a case wherelightly doped region 89 is P-type the actual PN junction of the diodewill be the interface between regions 89 and 90, whereas in a case wherelightly doped region 89 is N-type the actual PN junction of the diodewill be the interface between regions 81 and 89. N-type collectorregions 85 and 86 extend down into the P-type layer 81 from the uppersurface of the semiconductor material. A collector metal electrode 87makes contact with these N-type collector regions at a collector contact88. The collector metal electrode 87 also serves as an anode metalelectrode 95 and makes contact with the P-type anode 83 of PN junction84 at a diode contact 96. The collector metal electrode 87 bridges overthe base metal electrode 80 as illustrated. An amount of insulativematerial 120 prevents the collector metal electrode 87 from makingelectrical contact with the underlying base metal electrode 80. Thisdouble metal electrode structure involving a collector metal electrodethat bridges over a base metal electrode allows the RBJT to have a lowerforward voltage V_(F) as compared to a single metal layer structureinvolving interdigitated base and collector electrodes.

A lightly doped layer 89 is disposed under the P-type region 81. AnN-type layer 90 is disposed under the lightly doped layer 89. A part 91of the N-type layer 90 serves as the emitter region of RBJT 73, whereasanother part 92 of the N-type layer 90 serves as the N-type cathode 92of PN junction 84. The entire bottom surface of the semiconductormaterial is covered with a layer of metal that serves as an emittermetal electrode 93 and as a cathode metal electrode 94. Note that eachdiode contact (a contact from metal electrode 87 and 95 down to a PNjunction below) appears as a circle in the top-down perspective of FIG.12. The parallel diode 74 illustrated as a symbol in FIG. 6 actuallycomprises many PN junctions, each having a separate circle-shaped diodecontact. PN junction 84 of FIG. 13 is one of these many PN junctions.These many PN junctions are distributed across the integrated circuit.This structure involving many PN junctions distributed across theintegrated circuit is referred to as a distributed diode structure. Thedistributed diode structure provides better heat balancing as comparedto a structure where the parallel diode is realized as a singlenon-distributed junction that is located in only one localized part ofthe integrated circuit.

FIG. 14 is a diagram of the fly-back converter 50 of FIG. 6 but with aspecific implementation of LFVR 66 shown. The implementation of LFVR 66of FIG. 14 involves a current transformer 97 and the RBJT/diodeintegrated circuit 78. Current transformer 97 includes a first winding98, a second winding 99, and a ferrite core 100. The number of turns ofthe first winding 98 is at least twice as large (for example, threetimes as large) as the number of turns of the second winding 99. Asindicated by the dots on the ends of the winding symbols in FIG. 14, thefirst and second windings 98 and 99 are wound with respect to oneanother such that an increase in current in the second winding resultsin an increase in current in the first winding.

In the example of FIG. 14, the only electrical circuit component in thepath of the collector current is an inductive element (the secondwinding of current transformer 97). There is no resistive or diode orvoltage drop current sense element disposed in the collector currentpath between the first package terminal 70 and the collector of RBJT 73.There is no semiconductor material disposed in the collector currentpath. Similarly, there is no resistive or diode or voltage drop currentsense element disposed in the current path between the emitter of RBJT73 and the second package terminal 71. There is no semiconductormaterial disposed in the emitter current path.

FIG. 15 is a set of waveform diagrams that shows voltages and currentsin the fly-back converter 50 of FIG. 14. Switch 61 is closed from timet₀ to time t₁, and is open from time t₁ to time t₂ and then to time t₀of the next switching cycle. When the switch 61 is closed energy isbeing stored in the transformer 63, and when the switch 61 is open theenergy is transferred from the transformer 63 to capacitor 67 and load65. When the switch 61 is open and current is flowing through thesecondary winding 64, the forward voltage across LFVR 66 is about 0.1volts for most of the time from time t₁ to time t₂.

FIG. 16 illustrates current flow from time t₂ to time t₀. Referencenumeral 101 identifies the split core of transformer 63. FIG. 17illustrates current flow from time t₀ to time t₁. FIG. 18 illustratescurrent flow from time t₁ to time t₂.

FIG. 19 is a simplified cross-sectional diagram of a conventionalbipolar junction transistor (BJT) structure. The low doped region 200 isthe biggest source of conduction loss. The major charge carriers in thelow doped region 200 are electrons from the emitter 201. Holes from thebase 202 can enter the low doped region 200, but because the emittervoltage is lower than the voltage of the collector 203, the holes movetoward the emitter 201. Because the supply of holes in the low dopedregion is weak and because charge neutrality must be maintained, it isdifficult for the density of charge carriers in the low doped region 200to be much higher than the doping concentration of the low doped region.Accordingly, the low doped region has a relatively high resistance. Therelatively high resistance increases energy loss when the conventionalBJT is conducting current at its rated current.

FIG. 20 is a simplified cross-sectional diagram of RBJT 73. The lowdoped region 89 has more charge carriers than the low doped region 200in the conventional BJT structure of FIG. 19. There are both holes andelectrons in low doped region 89. Holes enter the low doped region fromthe base 82, and electrons enter the low doped region from the emitter91. Because the charges of these charge carriers are opposite oneanother, the charges cancel each other and net charge neutrality in lowdoped region 89 is maintained. Charge carrier density in the low dopedregion 89 is substantially higher than the doping concentration of thelow doped region. Accordingly, the low doped region has a relatively lowresistance and this relatively low resistance helps keep energy loss lowwhen the RBJT is conducting current at its rated current.

FIG. 21 is a waveform diagram for a conventional diode operating as therectifier component in a flyback converter from time t₁ to time t₂. Thetime from time t₁ to time t₂ is the time when current flows through thesecondary winding of the transformer and through the rectifyingcomponent. After an initial transient period, the voltage drop acrossthe forward biased diode decreases from about 1.0 volts to about 0.7volts at the end of the time period.

FIG. 22 is a waveform diagram for RBJT 73 of LFVR 66 of FIG. 14. Afteran initial transient period from t₁ to time t_(1A), the V_(CE) voltagedrop across RBJT 73 decreases from about 0.1 volts to about 0.05 volts.During this time from time t_(1A) to time t_(1B), the collector currentI_(C) and the base current I_(B) decrease as illustrated in the upperwaveform. At some point, the collector current reaches a criticalcurrent I_(C-CRIT) at which the base current I_(B) is not adequate tokeep the RBJT in saturation. This is indicated to occur at time t_(1B),in FIG. 22. At this time, V_(CE) begins increasing. At time t_(1C) theforward bias across diode 74 is large enough that diode 74 beginsconducting appreciable current. Diode 74 limits the voltage V_(LFVR) toabout 0.8 volts. At time t₂ current flow through the secondary winding64 stops, so current flow through LFVR 66 stops as well.

FIG. 23 is a table that compares the average power dissipation of theconventional flyback converter circuit of FIG. 1 (20 amperes out at 5.0volts DC) involving a conventional silicon diode as the rectifyingcomponent to the average power dissipation of the novel flybackconverter circuit of FIG. 6 (20 amperes out at 5.0 volts DC) involvingLFVR 66 as the rectifying component. Other than the type of rectifyingcomponent, the circuit topologies of the circuits of FIG. 1 and FIG. 6are identical.

FIG. 24 is a more detailed diagram of a particular implementation of theLFVR 66 of FIG. 14. A first end 102 of first winding 98 and a first end103 of second winding 99 are coupled together and to a transformerterminal 104. A second end 105 of first winding 98 is coupled to atransformer terminal 106. A second end 107 of second winding 99 iscoupled to a transformer terminal 108. The first winding 98 has threeturns. The second winding 99 has one turn. The terminal 104 of thetransformer is coupled via bond wire 109 to the first package terminal70 of LFVR 66. The terminal 106 of the transformer is coupled via bondwire 110 to a base terminal 111 of integrated circuit 78. The terminal108 of the transformer is coupled via bond wire 112 to a collectorterminal 113 of integrated circuit 78. An emitter terminal 114 ofintegrated circuit 78 is coupled via conductive heat sink 115 to secondpackage terminal 71. The conductive heat sink 115 is a portion of themetal leadframe of the package. The second package terminal 71 is apin-shaped extension of the heat sink. Both the pin-shaped secondpackage terminal 71 and the conductive heat sink 115 are stamped fromthe same piece of leadframe metal. In the diagram, A identifies firstpackage terminal 70: B identifies second package terminal 71; C, D and Eidentify the three terminals of current transformer 97; F, G and Hidentify the three terminals of integrated circuit 78. In theillustrated example, LFVR 66 has no package terminal other than the twopackage terminals 70 and 71.

FIG. 25 is a simplified perspective conceptual diagram of currenttransformer 97.

FIG. 26 is a more detailed perspective diagram of current transformer100. An insulative spacer 116 attaches to the bottom of the ring-shapedcore 100 as illustrated. The ring-shaped core 100 has bridging portions100A and 100B that allow turns of wire to loop under and around the corebetween the bottom of the bridge portions and the top of the spacer 116.In one example, insulative spacer 116 is molded plastic. In anotherexample, insulative spacer 116 is heat conductive ceramic. In anotherexample, insulative spacer 116 two-sided insulative tape.

FIG. 27 is a simplified perspective view of the packaged Low ForwardVoltage Rectifier (LFVR) 66. The extension 117 of the stamped and formedcopper leadframe is cut off. A piece of insulative tape 118 is disposedunderneath the transformer 97 between the transformer 97 and conductiveheat sink 115. In this case the insulative tape 118 is optional due toinsulative spacer 116 being present.

FIG. 28 is a perspective view of the packaged LFVR 66 after the assemblyhas been overmolded with an injection molded plastic encapsulant 119.The packaged LFVR 66 conforms to the form factor of a standard largeform factor through hold TO-247 package except that the middle terminalof the standard TO-247 package is not present.

FIG. 29 is a flowchart of a method 300 of manufacturing a packagedelectronic device. A LFVR (Low Forward Voltage Rectifier) is fabricated(step 301) by assembling a current transformer, a bipolar transistor,and a parallel diode such that the current transformer supplies anadequate base current to the transistor (for example, I_(B)=I_(C)/3)when current flows (a current greater than I_(C-CRIT) flows underforward bias conditions) from the first package terminal of the LFVR tothe second package terminal of the LFVR, thereby causing the transistorto have a collector-to-emitter voltage substantially less than 0.7 volts(for example, 0.1 volts). In one example, the assembled LFVR is LFVR 66illustrated in FIGS. 24, 27 and 28. The bipolar transistor is RBJT 73.The parallel diode is distributed diode 74. The RBJT and the distributeddiode of the assembly are parts of the integrated circuit 78. The firstpackage terminal is package terminal 70. The second package terminal ispackage terminal 71. Assembly involves surface mounting the currenttransformer and the integrated circuit to the heat sink portion of theleadframe, and then wire bonding the components together as illustratedin FIG. 27, and then overmolding the components to realize the finishedpackaged electronic device illustrated in FIG. 28.

FIG. 30 is a top-down view of RBJT/diode integrated circuit 78. Aportion of base metal electrode 80 is not covered by passivation andserves as base terminal 111. Base terminal 111 is a bond pad for wirebonding in this example. A portion of collector metal electrode 87 isnot covered by passivation and serves as collector terminal 113.Collector terminal 113 is a bond pad for wire bonding in this example.The third terminal 114 of the integrated circuit is disposed on thebottom side of the integrated circuit and is therefore not shown in FIG.30. Where the square portion 77 of FIG. 12 is located in the diagram ofFIG. 30 is identified with reference numeral 77.

FIG. 31 is a diagram that includes at the bottom a cross-sectional sideview of the rectangle portion 122 of the integrated circuit shown inFIG. 30, and also includes at the top a cross-sectional top-down view ofthe rectangle portion 122. The cross-sectional side view at the bottomof FIG. 31 is taken along the sectional line B-B in the top view of FIG.31. The cross-sectional top-down view at the top of FIG. 31 is takenalong the sectional line C-C in the bottom view of FIG. 31.

As illustrated in the top view of FIG. 31, each of the N-type collectorregions has a peripheral boundary which when viewed from the top-downperspective has a substantially square shape. Each of thesesubstantially square-shaped N-type collector regions has one and onlyone hole. This hole is a central axial hole. The N-type collector regionis therefore annular. The annular N-type collector region has a width ofabout 10.5 microns, and the diameter of the central hole is about 0.6microns. The P-type material of P-type region 81 extends up through thiscentral hole to the upper surface 123 of the semiconductor material. Theboundary between the bottom of collector metal electrode 87 and the topof P-type region 81 at the top of this hole is a diode contact. There isone such diode contact located in the center of each N-type collectorregion. Collector metal electrode 87 extends over the central part ofeach of these N-type collector regions as shown thereby establishing acollector contact with the underlying N-type collector region. Thecollector metal electrode 87 also extends over the hole therebyestablishing the diode contact to the corresponding underlying PNjunction of the distributed diode. The collector metal electrode 87extends from a collector contact with one collector region, up and overpart of the grid-shaped base metal electrode 80, and down to thecollector contact with a neighboring collector region. The N-typecollector regions in the center portion of integrated circuit 78 are allinterconnected by bridging collector electrode metal in this way. Thisbridging structure reduces the collector-to-emitter forward voltage ofthe bipolar transistor when the transistor is on.

When current is flowing through the distributed diode 74, current flowsfrom collector metal electrode 87, down through the many holes in N-typecollector regions, through the anode portions of P-type region 81,through lightly doped layer 89, through the cathode portions of N-typelayer 90, and to electrode 93. Underneath each hole (axial hole in anN-type collector region) is what is referred to generally here as a “PNjunction” even though it is understood that these PN junctions are partsof one larger PN junction. Reference numeral 84 identifies one such “PNjunction”. Reference numeral 129 identifies another such “PN junction”.All these PN junctions of the integrated circuit together constitute thedistributed diode 74 that is shown symbolically in FIG. 24.

Reference numeral 124 in FIG. 31 identifies the semiconductor portion ofthe integrated circuit 78. Lightly doped layer 89 is disposed overN-type emitter layer 90. In this example, lightly doped layer 89 has athickness of about 1.5 microns. These layers 89 and 90 extend all theway across the integrated circuit 78 in the lateral dimension. P-typeregion 81 extends down into the semiconductor portion 124 fromsubstantially planar upper surface 123 as shown. The substantiallysquare-shaped N-type collector regions extend down into P-type region 81from upper surface 123. Two of the N-type collector regions 85 and 86are identified with reference numerals in FIG. 31. Guard rings 125, 126and 127 of P-type material extend in parallel around the outer peripheryof integrated circuit 78.

FIG. 32 is another diagram of the rectangle portion 122 of FIG. 30. Thecross-sectional side view at the bottom of FIG. 32 is taken along thesectional line B-B in the top view of FIG. 32. The cross-sectionaltop-down view at the top of FIG. 32 is taken along the sectional linesD-D and E-E in the bottom view of FIG. 31. Each N-type collector regionin the center portion of integrated circuit 78 is entirely surrounded bythe base metal electrode 80. From a top-down perspective, the basecontact between the base metal electrode 80 and the underlyingsemiconductor material (at surface 123) has a two-dimensional gridstructure of horizontally extending strips and vertically extendingstrips. Within each of the squares formed by this two-dimensional grid,and extending down into the semiconductor material from surface 123, isone of the substantially square-shaped N-type collector regions. In thisexample, the width of the base contact is about 1.5 microns. The lateraldistance between adjacent N-type collector regions is about 2.0 microns.

FIG. 33 is another diagram of the rectangle portion 122 of FIG. 30. Thecross-sectional side view at the bottom of FIG. 33 is taken along thesectional line B-B in the top view of FIG. 33. The cross-sectionaltop-down view at the top of FIG. 33 is taken along the sectional lineF-F in the bottom view of FIG. 33.

FIG. 34 is a table that sets forth doping concentrations in the variousregions of the structure of FIG. 30.

FIG. 35 is a simplified flowchart of a method 400 of manufacturing anRBJT/diode integrated circuit in accordance with one novel aspect. Alightly doped layer is formed (step 401) over an N-type emitter layer. AP-type region is formed (step 402) to extend down into the lightly dopedlayer from an upper surface of the semiconductor portion of theintegrated circuit. A plurality of N-type collector regions are formed(step 403) to extend down into the P-type region from the upper surface.Each N-type collector region has a central hole. P-type semiconductormaterial from the underlying P-type region extends up into the hole fromthe bottom and to the upper surface. An emitter metal electrode isformed (step 404) on a side (bottom side) of the N-type emitter layeropposite the lightly doped layer. A two-dimensional grid-shaped basemetal electrode is formed (step 405) to surround each N-type collectorregion when the integrated circuit is considered from a top-downperspective. A collector metal electrode is formed (step 406) so thatthe collector metal electrode contacts each N-type collector region andalso extends over the hole in the center of the N-type collector region,thereby forming a diode contact in the center of each N-type collectorregion. The collector metal electrode also bridges up and overintervening portions of the base metal electrode to interconnect all theN-type collector regions. In one example, the integrated circuitstructure manufactured in the method 400 is the RBJT/diode integratedcircuit 78 structure pictured in FIGS. 30-33. The references above andin FIG. 35 to extending “over, to extending “down”, to the “upper”surface, to the “underlying” P-type region, and so forth are used todescribe relative orientations between different parts of the structurebeing described, and it is to be understood that the overall structurebeing described can actually be oriented in any way in three-dimensionalspace. The steps set forth above in FIG. 35 can be performed in anyorder so long as the same desired RBJT/diode integrated circuit 78results.

FIG. 36 is a simplified circuit diagram of a flyback power supplyconverter 500 in accordance with another novel aspect. The flybackconverter 500 is similar to the flyback converter 50 of FIG. 14, exceptthat the current transformer 97 and the transformer 63 of the circuit ofFIG. 14 are replaced with a three-winding transformer 501. Transformer501 has a primary winding 502, a first secondary winding 503, and asecond secondary winding 504. The number of turns of primary winding502, first secondary winding 603, and second secondary winding 604, are15, 1 and 1, respectively. Reference numeral 505 denotes a magnetic coreand magnetic coupling. The first end 506 of the primary winding 502 iscoupled to node 60. The second end 507 of primary winding 502 is coupledto switch 61.

The first end 508 of first secondary winding 503 is coupled to terminal111 of RBJT/diode integrated circuit 78 so that it can supply a currentI_(S1) to the base of bipolar transistor 73. The second end 509 of firstsecondary winding 503 is coupled to ground node 69. The first end 510 ofsecond secondary winding 504 is coupled to terminal 113 of RBJT/diodeintegrated circuit 78 so that it can supply a current I_(S2) to thecollector of bipolar transistor 73 and to the anode of diode 74. Thesecond end 511 of second secondary winding 504 is coupled to the secondend 509 of first secondary winding 503. Symbol 67 represents a storagecapacitance which may involve one capacitor or several capacitors. If asingle capacitor is used, then its leads need not be directly connectedto nodes 68 and 69 but rather there may be some intervening circuitrysuch as current sense or voltage sense circuits.

The bipolar transistor 73 is the only semiconductor component in acurrent path from the first end 508 of the first secondary winding 503to the output node 68. There is no semiconductor component in a currentpath between the first end 510 of the second secondary winding 504 andthe collector of RBJT 73. There is no semiconductor component in acurrent path between the emitter of RBJT 73 and output node 68.

In operation, switch 61 is rapidly switched on and off as in the exampleof FIG. 14 described above. Closing switch 61 causes a pulse of currentto flow through the primary winding 502. This pulse of current has asubstantially triangular shape as illustrated in FIG. 15. When switch 61is opened, current flow in the primary winding stops and current beginsto flow in the first and second secondary windings 503 and 504. Thewaveform of each of these currents I_(S1) and I_(S2) is of a triangularshape approximately as illustrated in the waveforms I_(B) and I_(C) inthe upper portion of FIG. 12. The transformer 501 controls the RBJT 73by supplying current I_(S1) such that RBJT 73 stays in saturationthroughout the first part of the off time of switch 61 as illustrated inthe bottom diagram of FIG. 22. As explained above in connection with theembodiment of FIG. 14, the forward voltage from terminal 113 to terminal114 across integrated circuit 78 is substantially lower than 0.7 volts.In one example, the forward voltage is about 0.1 volts to 0.05 volts.Over time, the magnitudes of the I_(S1) and I_(S2) current decrease tozero as illustrated in the waveforms labeled I_(B) and I_(C) in FIG. 22.

FIG. 37 is a diagram of one example of transformer 501. FIG. 38 showstransformer 501 in further detail. The core 505 involves two E-shapedferrite portions 512 and 513. These two E-shaped portions 512 and 513may be separated by an air gap as shown. The two E-shaped portions formthree branch portions 514, 515 and 516. The primary winding 502 iswrapped around the first branch portion 514. The first secondary winding503 is wrapped around the second branch portion 515. The secondsecondary winding 504 is wrapped around the third branch portion 516. Inthe example of FIG. 38, each of the three branch portions has an airgap. The ratio of the cross-sectional area at the air gap of portion514, to the cross-sectional area at the air gap of portion 515, to thecross-sectional area at the air gap of portion 516, is 4:1:3. Thenotation “AGA_(S1)=1” used in FIG. 38 indicates that the Air Gapcross-sectional Area (AGA) of the “S1” branch portion is one unit.Making the air gap cross-sectional area of the S2 branch portion threetimes as large as the air gap cross-sectional area of the S1 branchportion results in current I_(S2) being approximately three times aslarge as current I_(S1). Although the center branch portion for theprimary winding is shown having an air gap, in some examples this centerbranch portion has no air gap.

In the illustrated example, transformer 501 has three windings, and hasno winding other than the three windings. In another example, however,one other auxiliary winding is provided to provide power for poweringthe primary switch driver circuitry. In other examples, the flybackpower supply may have additional secondary windings to provideadditional output voltages.

FIG. 39 is a diagram of another example of the flyback power supply ofFIG. 36, but in this example the primary winding 502, the firstsecondary winding 503 and the second secondary winding 504 are allwrapped around the center branch portion 517 of the core 518. Core 518includes two E-shaped portions 519 and 520. The turns of wire of each ofthe three windings form a substantially cylindrical shape having anaxis, and the axes of these three windings are colinear with respect toone another. In the illustrated example, current I_(S1) is approximatelyequal to I_(S2). All branch portions of the core can have air gaps, oronly the center branch portion of the core can have an air gap, or theleft and right branch portions of the core can have air gaps without thecenter branch portion having an air gap.

FIG. 40 is a diagram of another example of the flyback power supply ofFIG. 36, but in this example the transformer functionality includes twotransformers 521 and 522. The first transformer 521 includes a firstprimary winding 523 and the first secondary winding. The current I_(P1)is the current flowing into the first primary winding 523. The secondtransformer 522 includes a second primary winding 524 and the secondsecondary winding. The current I_(P2) is the current flowing into thesecond primary winding 524. The transformers are interconnected andwound as shown in FIG. 40 such that the current waveforms of FIGS. 15and 22 are achieved.

FIG. 41 is a flowchart of a method 600 in accordance with another novelaspect. A first current is switched (step 601) so a pulse of a firstcurrent flows through a primary winding of a transformer. Thetransformer includes the primary winding, a first secondary winding anda second secondary winding. In one example, the pulse of the firstcurrent has a pulse shape illustrated in FIG. 15 as waveform I_(S).

A pulse of a third current is supplied (step 602) from the first end ofthe second secondary winding to the collector of a bipolar transistor.An anode of a diode is coupled to the collector of the bipolartransistor. A cathode of the diode is coupled to the emitter of thebipolar transistor. In one example, the pulse of the third current has apulse shape illustrated in FIG. 22 as waveform I_(C).

A pulse of a second current is supplied (step 603) from a first end ofthe first secondary winding of the base of the bipolar transistor. Inone example, the pulse of the second current has a pulse shapeillustrated in FIG. 22 as waveform I_(B).

As a result of the pulse of the third current and the pulse of thesecond current, a pulse of a fourth current flows from the emitter ofthe bipolar transistor. The pulse of fourth current flowing from theemitter is used (step 604) to charge a storage capacitance. In oneexample of the method 600, the storage capacitance is capacitance 67 inFIG. 36. Current flows and voltages are present as indicated in thewaveforms of FIG. 15. Within integrated circuit 78, part of this fourthcurrent flows into the base and out of the emitter of the bipolartransistor. Within integrated circuit 78, another part of this fourthcurrent may flow through the diode such as, for example, between timest_(1b) and t₂ as illustrated in FIG. 22. Regardless of how parts of thefourth current flow through the integrated circuit 78, the fourthcurrent flows from terminal 114 of the integrated circuit at the emitterof RBJT 73, at output node 68. This fourth current is used to charge andmaintain charge on storage capacitance 67 such that 5.0 volts DC ismaintained on storage capacitance 67.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Although the LFVR is described above in an applicationinvolving a specific flyback converter power supply, the LFVR is ofgeneral applicability in other circuits including other power supplycircuit topologies. Although an example is set forth above where thebase current injection circuit is implemented using a currenttransformer, the base current injection circuit can be implemented inother ways. Although the RBJT/diode integrated circuit is describedabove in connection an example in which the bipolar transistor is a NPNtransistor, in another example the bipolar transistor of the RBJT is aPNP transistor. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A flyback power supply circuit comprising: anoutput node; a ground node; a storage capacitance coupled between theoutput node and the ground node; a bipolar transistor having a base, anemitter and a collector, wherein the emitter is coupled to supply chargeto the storage capacitance via the output node; a diode having an anodecoupled to the collector of the bipolar transistor and having a cathodecoupled to the emitter of the bipolar transistor; a transformer having aprimary winding, a first secondary winding, and a second secondarywinding, wherein the first secondary winding has a first end and asecond end, wherein the second secondary winding has a first end and asecond end, wherein the first end of the first secondary winding iscoupled to supply current to the base of the bipolar transistor, andwherein the first end of the second secondary winding is coupled tosupply current to the collector of the bipolar transistor; and a switchcoupled to conduct current through the primary winding when the switchis closed and to prevent current flow through the primary winding whenthe switch is open.
 2. The flyback power supply circuit of claim 1,wherein the second end of the first secondary winding is coupled to thesecond end of the second secondary winding.
 3. The flyback power supplycircuit of claim 1, wherein the second end of the first secondarywinding is coupled to the ground node, and wherein the second end of thesecond secondary winding is coupled to the ground node.
 4. The flybackpower supply circuit of claim 1, wherein the transformer furthercomprises a core, wherein the core comprises a first portion about whichthe first primary winding is wrapped, wherein the core comprises asecond portion about which the first secondary winding is wrapped,wherein the core comprises a third portion about which the secondsecondary winding is wrapped, wherein the first portion of the core hasa first cross-sectional area, wherein the second portion of the core hasa second cross-sectional area, wherein the third portion of the core hasa third cross-sectional area, and wherein the second cross-sectionalarea is smaller than the third cross-sectional area.
 5. The flybackpower supply circuit of claim 1, wherein the primary winding has asubstantially cylindrical shape and has a first axis, wherein the firstsecondary winding has a substantially cylindrical shape and a secondaxis, wherein the second secondary winding has a substantiallycylindrical shape and a third axis, and wherein the first, second, andthird axes are colinear.
 6. The flyback power supply circuit of claim 1,wherein opening the switch during an operation of the flyback powersupply circuit results in a flow of a first current in the firstsecondary winding, and also results in a flow of a second current in thesecond secondary winding.
 7. The flyback power supply circuit of claim6, wherein a magnitude of the second current is a multiple of amagnitude of the first current.
 8. The flyback power supply circuit ofclaim 1, wherein the bipolar transistor is the only semiconductorcomponent in a current path from the first end of the first secondarywinding to the output node.
 9. The flyback power supply circuit of claim1, wherein there is no semiconductor component disposed in a currentpath from the first end of the second secondary winding to the collectorof the bipolar transistor.
 10. The flyback power supply circuit of claim1, wherein the diode and the bipolar transistor are parts of anintegrated circuit, and wherein the diode is a distributed diode.
 11. Amethod comprising: switching a first current flow through a primarywinding of a transformer of a flyback converter on and off, wherein thetransformer comprises the primary winding, a first secondary winding,and a second secondary winding; supplying a third current from a firstend of the second secondary winding to a collector of a bipolartransistor; supplying a second current from a first end of the firstsecondary winding to a base of the bipolar transistor such that when thefirst current is switched off during an operation of the flybackconverter the third current flows from the first end of the secondsecondary winding and to the collector of the bipolar transistor, andsuch that when the first current is switched off during the operation ofthe flyback converter the second current flows from the first end of thefirst secondary winding to the base of the bipolar transistor; and usinga current flowing from an emitter of the bipolar transistor to charge astorage capacitance, wherein the storage capacitance is coupled betweenthe emitter of the bipolar transistor and a ground node, wherein theground node is coupled to a second end of the first secondary windingand is also coupled to a second end of the second secondary winding,wherein an anode of the diode is coupled to the collector, and wherein acathode of the diode is coupled to the emitter.
 12. The method of claim11, wherein there is no semiconductor component in a current path fromthe first end of the second secondary winding to the collector of thebipolar transistor, and wherein there is no semiconductor component in acurrent path from the emitter of the bipolar transistor to the storagecapacitance.
 13. The method of claim 11, wherein the diode and thebipolar transistor are parts of an integrated circuit, and wherein thediode is a distributed diode.
 14. A flyback power supply circuitcomprising: a bipolar transistor having a base, an emitter and acollector; a diode having an anode coupled to the collector of thebipolar transistor and having a cathode coupled to the emitter of thebipolar transistor; a switch coupled to conduct pulses of a primarycurrent; means for receiving the pulses of the primary current and foroutputting corresponding pulses of a secondary current to the collectorof the bipolar transistor, wherein the means is also for driving thebase of the bipolar transistor such that the bipolar transistor ismaintained in saturation throughout at least most of the duration ofeach of the pulses of the secondary current; and an output storagecapacitor structure having a lead coupled to the emitter of the bipolartransistor.
 15. The flyback power supply circuit of claim 14, whereinthe means comprises a transformer, and wherein the transformer comprisesa primary winding, a first secondary winding, and a second secondarywinding.
 16. The flyback power supply circuit of claim 15, wherein theswitch is coupled to the primary winding of the transformer, wherein afirst end of the first secondary winding is coupled to the base of thebipolar transistor, and wherein a first end of the second secondarywinding is coupled to the collector of the bipolar transistor.
 17. Theflyback power supply circuit of claim 16, wherein the transformerfurther comprises a core, wherein the core comprises a first portionabout which the first primary winding is wrapped, wherein the corecomprises a second portion about which the first secondary winding iswrapped, wherein the core comprises a third portion about which thesecond secondary winding is wrapped, wherein the first portion of thecore has a first cross-sectional area, wherein the second portion of thecore has a second cross-sectional area, wherein the third portion of thecore has a third cross-sectional area, and wherein the secondcross-sectional area is smaller than the third cross-sectional area. 18.The flyback power supply circuit of claim 16, wherein the primarywinding has a substantially cylindrical shape and has a first axis,wherein the first secondary winding has a substantially cylindricalshape and a second axis, wherein the second secondary winding has asubstantially cylindrical shape and a third axis, and wherein the first,second, and third axes are colinear.
 19. The flyback power supplycircuit of claim 14, wherein a DC voltage is maintained on the outputstorage capacitor structure by pulses of current flowing from the means.20. The flyback power supply circuit of claim 16, wherein the means is atransformer, wherein there is no semiconductor component in a currentpath between the first end of the first secondary winding and the baseof the bipolar transistor, and wherein there is no semiconductorcomponent in a current path between the first end of the secondsecondary winding and the collector of the bipolar transistor.